The present invention relates to a novel method for thermal spray coating and a rare earth oxide powder used therefor or, more particularly, to a method for thermal spray coating capable of giving a highly heat-resistant, abrasion-resistant and corrosion-resistant coating layer on the surface of a variety of substrates and a rare earth oxide powder having unique granulometric parameters and suitable for use as a thermal spray coating material.
The method of so-called thermal spray coating utilizing a gas flame or plasma flame is a well established process for the formation of a coating layer having high heat resistance, abrasion resistance and corrosion resistance on the surface of a variety of substrate articles such as bodies made from metals, concrete, ceramics and the like, in which a powder to form the coating layer is ejected or sprayed as being carried by a flame at the substrate surface so that the particles are melted in the flame and deposited onto the substrate surface to form a coating layer solidified by subsequent cooling.
The powder to form the coating layer on the substrate surface by the thermal spray coating method, referred to as a thermal spray powder hereinafter, is prepared usually by melting a starting material in an electric furnace and solidifying the melt by cooling followed by crushing, pulverization and particle size classification to obtain a powder having a controlled particle size distribution suitable for use in the process of thermal spray coating.
A typical industrial field in which the method of thermal spray coating is widely employed is the semiconductor device manufacturing process which in many cases involves a plasma etching or plasma cleaning process by using a chlorinexe2x80x94and/or fluorine-containing etching gas utilizing the high reactivity of the plasma atmosphere of the halogen-containing gas. Examples of the fluorine- and/or chlorine-containing gases used for plasma generation include SF6, CF4, CHF3, ClF3, HF, Cl2, BCl3 and HCl either singly or as a mixture of two kinds or more. Plasma is generated when microwaves or high-frequency waves are introduced into the atmosphere of these halogen-containing gases. It is therefore important that the surfaces of the apparatus exposed to these halogen-containing gases or plasma thereof are highly corrosion-resistant. In the prior art, members or parts of such an apparatus are made from or coated by thermal spray coating with various ceramic materials such as silica, alumina, silicon nitride, aluminum nitride and the like in consideration of their good corrosion resistance.
Usually, the above mentioned ceramic materials are used in the form of a thermal spray powder prepared by melting, solidification, pulverization and particle size classification of the base ceramic material as a feed to a gas thermal spray or plasma thermal spray coating apparatus. It is important here that the particles of the thermal spray powder are fully melted within the gas flame or plasma flame in order to ensure high bonding strength of the thermal spray coating layer to the substrate surface.
It is also important here that the thermal spray powder has good flowability in order not to cause clogging of the feed tubes for transportation of the powder from a powder reservoir to a thermal spray gun or the spray nozzle because smoothness of the powder feeding rate is a very important factor affecting the quality of the coating layer formed by the thermal spray coating method in respect of the heat resistance, abrasion resistance and corrosion resistance. In this regard, the thermal spray powders used in the prior art are generally unsatisfactory because the particles have irregular particle configuration resulting in poor flowability with a large angle of repose so that the feed rate of the powder to the thermal spray gun cannot be increased as desired without causing clogging of the spray nozzle so that the coating process cannot be conducted smoothly and continuously greatly affecting the productivity of the process and quality of the coating layer.
With an object to obtain a thermal spray coating layer having increased denseness and higher hardness, furthermore, a method of reduced-pressure plasma thermal spray coating is recently proposed in which the velocity of thermal spraying can be increased but the plasma flame is necessarily expanded in length and cross section with a decreased energy density of the plasma flame so that, unless the thermal spray powder used therein has a decreased average particle diameter, full melting of the particles in the flame cannot be accomplished. While a thermal spray powder having a very small average particle diameter is prepared, as is mentioned above, by melting the starting material, solidification of the melt, pulverization of the solidified material and particle size classification, the last step of particle size classification by screening can be conducted only difficulties when the average particle diameter of the powder is already very small.
While in the prior art, many of the parts or members of a semiconductor-processing apparatus are made from a glassy material or fused silica glass, these materials have only low corrosion resistance against a plasma atmosphere of a halogen-containing gas resulting not only rapid wearing of the apparatus but also a decrease in the quality of the semiconductor products as a consequence of surface corrosion of the apparatus by the halogen-containing plasma atmosphere.
Although ceramic materials such as alumina, aluminum nitride and silicon carbide are more resistant than the above mentioned glassy materials against corrosion in a plasma atmosphere of a halogen-containing gas, a coating layer of these ceramic materials formed by the method of thermal spray coating is not free from the problem of corrosion especially at an elevated temperature so that semiconductor-processing apparatuses made from or coated with these ceramic materials have the same disadvantages as mentioned above even if not so serious.
The present invention accordingly has an object, in order to overcome the above described problems and disadvantages in the prior art methods of thermal spray coating, to provide a novel and improved method of thermal spray coating which can be conducted at a high productivity of the process by using a thermal spray powder having excellent flowability in feeding and good fusibility in the flame and capable of giving a coating layer with high corrosion resistance against a halogen-containing gas or a plasma atmosphere of a halogen-containing gas even at an elevated temperature.
Thus, the present invention provides a method for the formation of a highly corrosion-resistant coating layer on the surface of a substrate by thermal spray coating, which comprises the step of: spraying particles of a rare earth oxide or a rare earth-based composites oxide, in which the impurity content of an iron group element or, in particular, iron does not exceed 5 ppm by weight calculated as oxide, at the substrate surface as being carried by a flame or, in particular, plasma flame to deposit a melt of the particles onto the substrate surface forming a layer. It is further desirable that the contents of alkali metal elements and alkaline earth metal elements as impurities in the rare earth oxide-based thermal spray powder each does not exceed 5 ppm by weight calculated as the respective oxides.
In particular, the particles of the rare earth oxide or rare earth-based composite oxide have an average particle diameter in the range from 5 to 80 xcexcm with a dispersion index in the range from 0.1 to 0.7 and a specific surface area in the range from 1 to 5 m2/g. More particularly, the particles are preferably granules of a globular configuration obtained by granulation of primary particles of the oxide having an average particle diameter in the range from 0.05 to 10 xcexcm.
It is more desirable that the above described rare earth oxide-based thermal spray powder has the granulometric characteristics including;
a globular particle configuration with an aspect ratio of the particles not exceeding 2;
a particle diameter D90 at 90% by weight level in the particle diameter distribution not exceeding 60 xcexcm;
a bulk density not exceeding 1.6 g/cm3; and
a cumulative pore volume of at least 0.02 cm3/g for the pores having a pore radius not exceeding 1 xcexcm.